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FLAXWBLT: A STUDY OF THE NATURE AND INHERIT- 
ANCE OF WILT RESISTANCE 



BY 



W. H. TISDALE 



Reprinted from JOURNAL OF AGRICULTURAL RESEARCH 

Vol. XI, No. 11 : : : Washington, D. C., December 10, 1917 




PUBLISHED BY AUTHORITY OF THE SECRETARY OF AGRICULTURE, WITH THE COOPERATION 
OF THE ASSOCIATION OF AMERICAN AGRICULTURAL COLLEGES AND EXPERIMENT STATIONS 



WASHINGTON : GOVERNMENT PRINTING OFFICE 1 1917 



: -■'Ri- : :^: ; :^m:;:h : y.ttS 









FLAXWILT: A STUDY OF THE NATURE AND INHER- 
ITANCE OF WILT RESISTANCE 

By W. H. Tisdale, 
Wisconsin Agricultural Experiment Station 

INTRODUCTION 

The main object of these investigations has been to study the nature 
and inheritance of wilt resistance. Bolley (4-11) 1 has so thoroughly 
worked out the more practical phases of the problem that it has left a 
clear field for a study of this kind. It seems important, however, to 
give, by way of introduction, a few fundamental statements regarding 
the disease and the causal organism. 

The flaxwilt problem has been of great importance in this country 
and is also a serious problem in some of the flax-growing countries of the 
East (5, 12, 19, p. 211-217). In America the flax industry has been 
forced rapidly westward, owing to the loss from wilt. The disease is 
typical of the wilt diseases which are produced by various species of 
Fusarium. Plants at any age from germination to maturity may be 
attacked and killed by the parasite. The disease is manifested by a 
sudden wilting of young seedlings and a yellowing of the foliage of older 
plants, followed by a wilting which may involve the entire plant or only 
one side of it, thus causing a bending or twisting of the plant toward 
the wilted side. The disease is highly destructive to common flax 
(Linum usitatissimum) when grown on thoroughly infected soil, and 
very often the entire crop is destroyed. 

SOURCE OF MATERIAL, 

Flaxseed and "flax-sick soil" were kindly supplied for this work by 
Prof. H. L. Bolley, 2 of the North Dakota Agricultural Experiment 
Station. From plants grown from these seeds in the "sick soil" a 
species of Fusarium was isolated which agreed in cultural characteristics 
and pathogenicity with Fusarium lini Bolley (4). There was a wider 
range of spore size than Bolley gave in his original description, but this 
could be accounted for by the difference in environmental conditions 
under which the spores were produced, as environment was found to 

1 Reference is made by number (italic) to " Literature cited," p. 604-605. 

8 The writer wishes to express his sincere appreciation to Prof. H. h. Bolley for his hearty cooperation 
in offering invaluable suggestions and in supplying material for the work. He is also indebted to Prof. 
L. R. Jones, of the Plant Pathology Department, and to Prof. h. J. Cole, of the Experimental Breeding 
Department, of the University of Wisconsin, for their aid in denning the problems at the outset and for 
their respective supervision and kindly criticisms of the pathological and breeding phases as they pro- 
gressed. 

Journal of Agricultural Research, Vol. XI, No. n 

Washington, D. C. Dec. 10, 191 7 

lb KeyWis.No.— 8 

(573) 



574 Journal of Agricultural Research vol. an, no. « 

have a considerable influence on spore size in pure cultures. This 
organism, which is no doubt identical with F. lini, is the only pathogenic 
form of Fusarium isolated from flax during this work. 

STRAINS OF FLAX USED 

In undertaking a study of the inheritance of wilt resistance one of the 
most important considerations was to obtain for the crosses strains of 
flax which were highly resistant and strains which were highly suscept- 
ible to the disease. A number of both resistant and susceptible strains 
were obtained from Prof. Bolley. Several varieties of common flax 
were also obtained from various places in North Dakota and Minnesota. 
Before the crossing work was begun all of these strains, or varieties, 
were thoroughly tested as to resistant and susceptible qualities on "flax- 
sick soil" from North Dakota. Plants for these tests, as well as for all 
infection experiments, were grown in the greenhouse. Some of the 
strains proved satisfactory under greenhouse conditions, while others did 
not. All of the resistant strains except North Dakota Resistant 1 14 were 
discarded, that variety being used exclusively, as it proved to be supe- 
rior in resistance to all other strains tested (PI. 44, B, a). This strain is 
designated as No. 4 throughout this work. Prof. Bolley in sending the 
seed of No. 4 says: 

This flax has been growing on the North Dakota State grounds for a number of 
years and ought to be highly resistant to wilt. 

The most satisfactory strain of susceptible flax used is strain 3 (North 
Dakota Pure-Seed Laboratory No. 14654; PI. 44, B, b). Bolley states 
that this strain died out completely on his seed plots at the North Dakota 
station and should be well suited for work of this kind. Two other 
strains of common flax have been used to some extent, but have not 
been so satisfactory as No. 3 in giving uniform results. One of these, 
No. 5 — a white-flowered variety(Pl. 45, B, b) — was obtained from a 
linseed mill at Red Wing, Minn. This was the only white-flowered variety 
tested, and, although it was found to be entirely susceptible, it does not 
wilt so rapidly and uniformly as No. 3. The other, No. 6 (PI. 45, A, d), 
is a strain of common flax obtained from Mr. M. S. Kirk, Devils Lake, 
N. Dak. It is similar to No. 3, but is slightly less susceptible. 

DEFINITION OF THE PROBLEM 

The first available remedy for controlling flaxwilt was introduced with 
Bolley's (4) discovery of the cause of the disease and his selection of 
flax plants for wilt resistance. The variation of such disease resistance 
of individual plants within a variety was the basis for Bolley's selection. 
This method of selection has likewise been successfully employed in 
obtaining disease-resistant strains of other cultivated plants (75, 18, 20, 
21). In connection with this improvement by selection, there has 






Dec. 10, 191 7 



Flaxwilt 575 



naturally arisen the question as to the nature of individual variation 
and the cause of disease resistance. Although there has been consider- 
able theorizing concerning the possible cause, or nature of resistance, 
very little intensive research has been directed toward a positive solu- 
tion of the problem. Biffen's (2, 3) hybridization experiments with 
wheat were the first to throw light on disease resistance as being due 
to inheritable factors which behave according to the laws of Mendel. It 
hardly seems possible, however, to explain the inheritance of such a char- 
acter as resistance on so simple a basis as that stated by Biffen. 

Having in mind these questions concerning the nature and inheri- 
tance of wilt resistance in flax, the writer undertook the present investi- 
gations with the Departments of Plant Pathology and Experimental 
Breeding at the University of Wisconsin in February, 1915, his objects 
being (1) to study the mode of penetration of flax plants by F. lint, (2) to 
make comparative studies on the penetration of cabbage seedlings by 
F. conglutinans (3) to determine whether or not F. lini enters the resistant 
flax plant, (4) to study the relation of the fungus to the tissues of suscepti- 
ble and resistant flax plants, and (5) to study the inheritance of wilt resist- 
ance through hybridization. Flax was found to be a most suitable 
plant for the hybridization work for a number of reasons — namely, it 
has a short growing season, can be grown to maturity in the greenhouse, 
is easily cross-pollinated, and highly resistant and susceptible strains 
are available. Even with these advantages it was not expected that 
more than a clue as to the nature of the inheritance of wilt resistance 
might be obtained in the time allotted for the work, since breeding is a 
slow process and it was necessary to develop methods as the work pro- 
gressed. 

NATURE OF WILT RESISTANCE 

MODE OF PENETRATION 

Before making a detailed study of the relation of the fungus to the 
various host tissues it was considered of fundamental importance to 
know how it enters the host, and to know whether or not it penetrates 
the resistant plant. There seems to be very little definite information 
as to the exact mode of entrance of the parasitic soil fungi into their 
hosts. It was hoped that by using pure-culture methods penetrations 
of flax and cabbage seedlings by species of Fusarium might be obtained 
so that they could be detected by aid of the microscope. Bolley (4) states 
that F. lini penetrates the young flax plants at any point, through the 
seed, leaves, stem, or roots. His illustration shows very clearly that 
the fungus is able to penetrate the cell walls at any point, but he does 
not show conclusively the initial points of entrance. However, he was 
dealing with the subject mainly from a practical rather than from a 
standpoint of detailed microscopical study. 



576 Journal of Agricultural Research vol. xt, No. » 



In order to make a careful study of penetration, special culture methods 
were necessary. Test tubes were prepared by placing in the bottom 
rolls of filter paper (PI. 44, A) which were moistened and rendered suit- 
able as a medium for fungus growth by pouring a small amount of 
melted potato agar over them. These tubes were autoclaved and then 
planted in equal numbers with seeds of both resistant and susceptible 
varieties of flax. These seeds had been previously treated for five 
minutes with a 1 -to- 1,000 solution of mercuric chlorid. At the time of 
planting, or in some cases just after the seeds had germinated, some 
of the tubes containing each strain were innoculated with F. lint from a 
pure culture, while others were left uninoculated to serve as controls, 
and in most cases these remained free from fungus growth (PI. 44, A, 
b, c). Cabbage seedlings were also grown for penetration studies in 
tubes prepared as above and inoculated with F. conglutinans from a pure 
culture. The potato agar served as an excellent medium for the growth 
of the species of Fusarium, which had immediate access to the young 
seedlings growing in the tubes. As soon as any signs of wilting could be 
seen, the seedlings were carefully removed from the tubes, mounted on 
slides in a 20 per cent glycerin solution and examined carefully under the 
microscope. In this way the root hairs can be easily observed, and 
almost the entire young root system is so transparent that penetrating 
hyphae can be detected. In some cases it was necessary to separate the 
cortical layer from the inner part of the root tissue in order to be able 
to examine it closely. This was done by carefully splitting the root on 
one side with a sharp scalpel and removing the cortical layer, which was 
then spread on the slide and mounted in glycerin, as mentioned above. 
Penetration studies were also made with young flax seedlings grown in 
very loose, infected soil. It is very difficult to obtain clean root hairs 
from plants grown in soil. The best results were secured by taking 
the plant up with a large lump of soil which was then dissolved away 
without destroying all of the root hairs by placing it in a vessel of still 
water and agitating gently. These roots were then mounted and exam- 
ined in the same manner as those taken from the tubes. A small amount 
of eosin placed under the cover slip was often of considerable aid in 
observing root hairs and hyphae. 

A careful study of slides prepared as above described revealed root- 
hair (fig. 1, G, H, I), epidermal (fig. 2, 3), and stomatal (fig. 4), pene- 
tration of flax seedlings taken from tube cultures; root-hair (fig. 1) 
and epidermal (fig. 3), penetration of flax seedlings grown in infected 
soil; and root hair penetration (fig. 1, A, B) of cabbage seedlings taken 
from tube cultures. The cabbage seedlings produce a more abundant 
supply of root hairs, and are, for this reason, more desirable for the 
study of root hair penetration than are flax seedlings. By cross inocu- 
lation in tube culture it was found that Fusarium conglutinans could 
penetrate the root hairs of flax seedlings (fig. 1, K). Likewise, F. lini 



Dec. 10, 1917 



Flaxwilt 



577 




Fig. i.— A-F, Fusarium conglutinans penetrating root hairs of cabbage seedlings in pure culture in test 
tubes. G , H , and l.F.lini penetrating root hairs of flax seedlings grown in test tube cultures. J , F. lint 
penetrating root hair of flax plant grown in loose, infected soil. K, F. conglutinans penetrating root hair 
of flax seedling in pure culture in test tube. Camera-lucida drawings. 



578 



Journal of Agricultural Research 



Vol. XI. No. ii 



was evidently able to penetrate cabbage seedlings as they were killed 
by it in tube cultures. However, no penetration was observed in the 
latter as examination was limited. When flax was planted on "cabbage- 
sick soil " and the reverse, no wilting or yellowing occurred. The fungus, 
very likely, enters the root hairs to some extent, but probably is unable 
to invade the tissues of the plant for the same reason that F. lini is unable 
to invade the tissues of the resistant flax plant. It was shown by the 




Fig. 2.—Fusarium lini penetrating epidermis of young flax root grown in loose, infected soil. 

tube-culture method (PI. 44, A, a, d) that F. lini can penetrate the 
young seedlings of the resistant strain of flax as readily as it can pene- 
trate the seedlings of the susceptible strain under those conditions. 

By what exact means the fungus is able to penetrate the cell walls 
of the host is not known. Perhaps the most feasible explanation is that 
given by Ward (26) that the fungus protoplasm overcomes the resistance 

of the cells of the host 
««-*«?" by means of enzyms 

or toxins. This might 
be interpreted as 
meaning that the fun- 
gus secretes an enzym 
which has a solvent 
action on the cell wall, 
or that it may secrete 
a toxin which prevents 
any reaction on the part 
of the host-cell proto- 
plasm by killing or weakening it, thus making possible the invasion of the 
cell. Perhaps both of these phenomena occur simultaneously. In the case 
of root-hair penetration there is a slight depression at the point of entrance 
of the fungus, and the diameter of the opening made by a hypha is some- 
what less than the regular diameter of the hypha in question. Stomatal 
penetration was found on the stem of a young flax seedling near the 
point where the root began branching. Seedlings taken from the soil 




Fig. 3.—Fusarium lini penetrating epidermis of young flax seedling 
in test-tube culture. 



Dec. lo. 1917 Flaxwilt 579 

showed stomata on the parts which were below the soil surface ; therefore 
it is possible that stomatal penetration is a source of infection as well as 
root-hair penetration, and the penetration of the epidermis of young 
roots. The fungus is also capable of infecting through wounds, as was 
shown by artificial inoculations (Table I). This discovery of root-hair, 
epidermal, and stomatal penetration bears out Bolley's assumption that 
the fungus is able to penetrate the young plant at any point. 

RELATION OF THE FUNGUS TO THE SUSCEPTIBLE PLANT 

The relation of fungi to their host tissues is a very complicated one 
which varies greatly, according to the fungus and host under considera- 
tion. The object of this work was to study the relation of the fungus 
to the various tissues of the host and to seek any evidence that might 
be of value in explaining the possible cause for certain disease phenomena. 




Fig. 4.— Fusarium lint entering stoma of young flax seedling in test tube culture. 

After entering the susceptible plant, the fungus passes directly through 
the cell walls of the parenchyma tissues to the vascular system, which 
it invades to its limits. Bolley (4) states that sections through the stems 
and roots of wilted plants show that the parasite is able to penetrate the 
cell walls at any point and pass directly through any of the tissues, not 
excepting the woody parts. This statement holds true especially for 
plants in the later stages of wilt. Very few fungus hyphae can be found 
in the cortical tissues of the stems of plants which have just wilted, 
although these tissues become thoroughly ramified with hyphae as the 
plant begins to decay. The cortical parenchyma of the roots, however, 
is the first tissue to be invaded by the fungus. In the early stages of the 
disease the hyphas are confined largely to the woody tissues in the stem. 

Eight newly wilted plants ranging from half-grown to the late-flower- 
ing stage were stripped of their foliage, treated for five minutes in a 
1 -to- 1, 000 solution of mercuric chlorid, washed in sterile water, cut in 
pieces with sterile instruments, and plated out on potato agar. Each 
piece was noted carefully, in order to be sure just what part of the plant 
it came from. Pure cultures of F. lini were obtained from all parts of 



5 8o 



Journal of Agricultural Research 



Vol. XI, No. ii 



the stems of six of these plants up to the terminal bud, and, in one case, 
growth was obtained from the seed capsules. The two other plants 
showed growth up to within i inch of the terminal bud. Sections taken 
from near the top of the stem of a plant which had just wilted, and stained 
with "Pianeze Illb stain" (24) showed the fungus hyphae in the vascu- 
lar system but not in the cortex. The method used in staining was 
varied somewhat from that described by Vaughan (24). The slides were 
passed from xylol through absolute and 95 per cent alcohol into the 
stain, where they were allowed to remain overnight. They were then 
washed rapidly in water to remove loose stain and detained in 95 per 
cent alcohol until the desired point was reached. After detaining they 
were passed through absolute alcohol and xylol and mounted in balsam 




Fig. 5.— Longitudinal section of woody tissues of susceptible flax plant showing the invading hyphae 

of Fusarium lini. 

in the regular manner. Excellent results were obtained by this method 
of staining. 

There seems to be very little reaction on the part of the protoplasm 
of the susceptible host toward checking the invasion by the fungus. The 
fungus grows rapidly within the tissues (fig. 5), and sometimes micro- 
conidia are produced in the vascular cells of the host plant (fig. 6). 
Some of the vessels may, in rare cases, become almost clogged with fungus 
hyphae (fig. 5), but this is so rare that it would hardly seem possible 
that the wilting could be due to the cutting off of the water supply by 
this means (fig. 7). Gilman (14) believes that the yellowing of cabbage 
is due to the slow drain made by Fusarium conglutinans on the water sup- 
ply combined with the high temperature, which causes an increased 



Dec. 10, 191 7 



Flaxwilt 



58i 



growth of the fungus and which increases the transpiration of the plant. 
This seems reasonable, so far as it goes, but does not seem sufficient for 
a complete explanation. If it were a case of the water supply being 
cut off by the clogging of the vessels, we should not expect so much of 
the one-sided wilting of plants which is so common with flax. The 
leaves on one side of the stem may become yellow, while those on the 
other side remain perfectly normal. Stems of plants which are wilted 
on one side only become characteristically twisted or curved, owing per- 
haps to unequal growth and shrinkage of tissues. If this wilting were 
due to the mere cutting off of the water supply at some point, it would 
be reasonable to except that when a normal plant has its stem cut half- 
through it would turn yellow and wilt on the cut side. Five flax plants 
were cut in this way, but none of them showed any yellowing or wilt- 
ing from the wound. It is certain that by the time the foliage of the 
plant begins to wilt the root system has been invaded rather severely 




Fig. 6. — Longitudinal section of the woody tissue of the susceptible flax plant showing the invading 
hyphae of Fusarium lint. Notice the microspores of the fungus in the host cells. 

by the fungus, and the root hairs are largely destroyed. This is espe- 
cially true in case of the flax, and would likely account to a considerable 
extent for any lack in the water supply, and for the general weakening of 
the plant. Furthermore, there must be a protoplasmic disturbance in 
cells of the invaded tissues, which helps to produce the local symptoms. 
Phenonema of this kind might be due to toxic substances produced by the 
fungus, which interferes with the normal functions of the host proto- 
plasm. The fungus also consumes a part of the food and water sup- 
ply of the plant. There are very likely, a number of factors which aid 
in the production of wilt symptoms due to the invasion of flax by F. lini — 
namely (1) Partial destruction of the root system which limits the food 
and water supply of the plant; (2) use of part of the food and water 
supply of the plant by the fungus; (3) an increase in transpiration and 
an increase in the growth of the fungus due to a rise in temperatures; 
and (4) the possible production of toxic substances by the fungus, which 
interferes with the normal functions of the host protoplasm. 



582 



Journal of Agricultural Research 



Vol. XI, No. ii 



RELATION OF THE FUNGUS TO THE RESISTANT PLANT 

After finding that the fungus was able to penetrate seedlings of the 
resistant strain of flax in tube cultures, experiments were planned to 
determine whether or not the fungus was entering the resistant plants 
growing in infected soil, and, if so, why it was not able to invade the plant 
and cause wilt. It was found by inoculation experiments that the fungus 
was unable to produce wilt in resistant plants when introduced through 
wounds, although infection was obtained by inoculating plants of the 
susceptible strain. Inoculations were made by inserting bits of mycelium 




Fig. 7. — Cross section of the vascular tissues of a susceptible flax plant, showing the invading hyphae of 
Fusarium lini. Camera-lucida drawing. 

into needle wounds in the stems of plants. Some were inoculated just 
above and others just below the soil surface. Stems inoculated above 
ground were wrapped with moist cotton, while those inoculated below 
the surface of the ground were covered by replacing the soil. Table I 
gives the results of these inoculations. It will be seen from this table 
that the inoculations made in the field were less successful than those 
made in the greenhouse. The plants in the field were in the flowering 
stage, which is rather late for infection, and were growing under conditions 
where the moisture could not be satisfactorily controlled. In the green- 
house young plants were inoculated and kept under better controlled 
moisture conditions. 



Dec. 10, 1917 



Flaxwilt 



583 



Table I. — Results of artificial inoculations of resistant and susceptible flax plants with 

Fusarium lini 



Date. 



Strain of flax. 



Number of 
plants in- 
oculated. 



Number of 

field plants 

infected. 



Number of 
greenhouse 
plants in- 
fected. 



August 7 . . . . 

Do 

August 17 . . . 

Do 

Do 

August 26 . . . 

Do 

Do 

November 29. 

Do 

Do 



1916. 
November 26. 
Do 



Resistant... 
Susceptible . 
Resistant. .. 
Susceptible . 

...do 

Resistant. . . 
Susceptible . 

do 

Resistant. .. 
Susceptible 
do 



Resistant. . . 
Susceptible . 



o 

!3 



Since no infection was obtained by inoculating plants of the resistant 
strain, while those of the susceptible strain did become infected, it seems 
that there must be some immediate reaction on the part of the protoplasm 
of the resistant plant to check invasion by the fungus. 

Whatever the nature of resistance, it is not manifested to the highest 
degree unless the plant is kept under perfectly normal conditions. This 
fact was shown by the way in which seedlings of the resistant strain of 
flax were killed by the fungus in tube cultures (PI. 44, A, d), and was 
further tested by planting disinfected seeds of both the resistant and 
susceptible strains in flasks of soil which had been sterilized and inocu- 
lated with F. lini. When these flasks were plugged with cotton and kept 
in the laboratory, the resistant strain of plants showed slightly more 
resistance at first, but later succumbed to the attack. However, when 
flasks were prepared as above and placed in the greenhouse without the 
cotton plugs, some of the plants of the resistant strain lived to the flower- 
ing stage, while plants of the susceptible strain immediately died of wilt. 
Even under greenhouse conditions, where the temperature runs above 
normal, some of the plants of the resistant strain wilt. 

Careful examination of the root system of resistant plants grown in 
infected soil showed that some of the smaller roots were decaying and 
there were brownish spots on the larger roots. A number of these plants 
which showed no signs of wilt above ground were taken and the root sys- 
tem disinfected thoroughly on the surface with a i-to-1,000 solution of 
mercuric chlorid for 2% to 5 minutes. They were then washed thoroughly 
and plated out on potato agar. In a large percentage of cases pure 
cultures of F. lini were obtained from these roots. In some cases other 



5§4 



Journal of Agricultural Research 



Vol. XI, No. i 



fungi appeared but not in so great an abundance as the parasitic form. 
Most of the root hairs and very small roots were left in the soil when 
these plants were removed; therefore the fungus obtained in these 
experiments came mainly from the larger roots, which were perhaps less 
likely to be penetrated than the smaller roots. Table II gives the results 
of these isolation experiments. It will be noticed that roots plated out 
on February 21, 191 6, gave a much lower percentage of F. lini than did 
the others. This is probably due to the fact that these plants were 
grown in midwinter, when the soil was cooler and the fungus less vigorous. 
Some of the plants of the resistant strain succumb to the disease in the 
summer, when the temperatures are high; therefore we should be more 
likely to find the fungus in the roots under summer conditions, as infec- 
tion is more abundant. 

Table II. — Isolation of Fusarium lini from roots of resistant flax plants 



Date. 



Oct. 4, 191 5. . 
Oct. 7, 1915. . 
Feb. 21, 1916. 
Sept. 20, 1916 
Sept. 26, 1916 



Period of 
treatment 
with mer- 
curic 
chlorid. 



Minutes. 
2K 

5 
5 
5 



Number of 
plant roots 
plated out. 



3 

7 

16 
12 

28 



Number of 
roots show- 
ing growth 
of F. lini. 



3 

7 

4 

12 

23 



This table shows that the fungus is at least able to penetrate deep 
enough beneath the surface of the resistant plant to protect it from the 
mercuric chlorid used in disinfecting the surface, but does not give any 
evidence as to the extent of invasion. 

Parts of the roots of resistant flax plants which showed brown-spotting 
were fixed in Flemming's medium fixative, embedded, sectioned, and 
stained with "Pianeze Illb stain" (24) as previously described. This 
stain gives a pink color with the parenchyma cell walls of the host and 
with the fungus tissues. With lignified, cutinized, and suberized tissues 
it gives a light green. A careful study of these sections showed that 
the fungus entered the parenchyma tissues of the resistant plant (fig. 8), 
but seldom, if ever, penetrated so far as the xylem elements. This 
limited invasion by the fungus is accompanied by a number of cellular 
changes on the part of the host tissues which are in the immediate 
vicinity of the invading hyphae. 

(1) There is a slight breaking down of the invaded cells which, how- 
ever, is not sufficient within itself to disconnect the hyphae from sur- 
rounding cells. Marryat (17) and Ward (25) state that in the case of 
wheat which resists yellow-rust (caused by Puccinia glumarum) there 
is a sudden breaking down of the first cells to be invaded, thereby cutting 



Dec. 10, 191 7 



Flaxwilt 



585 




Fig. 8. — A, Longitudinal section of the cortical parenchyma of a resistant flax root showing the formation 
of cork walls around the pointof invasion by Fusarium lint. B, Longitudinal section of the cortical paren- 
chyma of a resistant flax root showing a cork layer formed between the point of invasion and the vascular 
system. Notice the increased cell division beneath the cork layer. C, Longitudinal section of the cortical 
parenchyma of a resistant flax root showing cell-wall penetration by F. lini and the formation of cork walls 
between the invading hypha and the vascular system of the root. The protoplasm in the cork cells was 
granular as indicated by stipling. D, Cross section of the cortical parenchyma of a resistant flax root 
showing the heavy cork walls formed around the point of invasion by F. lini. Camera-lucida drawings. 



586 Journal of Agricultural Research vol. xi, No. » 

the fungus off from connection with other cells of the host. The fungus 
then dries up and dies or remains dormant in the dead host cells. The 
resistant flax plant behaves differently toward F. lint than does wheat 
toward P. glumarum. The rust fungus is an obligate parasite and is not 
capable of growing in dead tissues ; F. lini may grow either as a parasite 
or as a saprophyte, and for this reason its development would not be 
checked by the death of the host cells. Furthermore, the breaking 
down of the cells in the flax plant is not so complete as that stated for 
wheat by Marryat and Ward, and would doubtless play very little part 
in decreasing further invasion by the fungus. 

(2) In some cases the protoplasm of the host cells immediately sur- 
rounding the point of invasion becomes granular in appearance and 
stains green with the Pianeze stain, whereas the protoplasm of the normal 
cell fails to take the green at all. The writer is unable to offer any defi- 
ite explanation for this change other than to say that it is possibly a 
chemical change in the host protoplasm excited by the presence of the 
fungus. There is doubtless a certain amount of injury to the host pro- 
toplasm, which may account in part for the coarse granular condition. 
Furthermore, as will be mentioned later, there may be some substance 
produced which is injurious to the fungus . 

(3) Surrounding the area in which the cells show the granular appear- 
ance, there is a stimulation to cell division. This cell division is more 
abundant toward the vascular system from the point of invasion. In 
some cases the dividing walls are formed more or less irregularly, while 
in other cases a typical cork cambium seems to be formed. The newly- 
formed cells are to all appearance cork cells. 

(4) Accompanying this cell division and other phenomena is a thick- 
ening of cell walls, which is much more noticeable toward the vascular 
system from the invaded point. This thickening of walls may extend 
three or four cell layers beyond the point of invasion and is more pro- 
nounced with newly formed cells. However, the walls of cells which 
were formed previous to invasion may become thickened. The process 
of thickening seems to be a laying down of additional material which is 
produced by the protoplasm of the affected cells. The modification of 
old walls is more noticeable below and above the point of invasion and 
toward the epidermis, where cell division is not abundant. These 
thickened walls stain green with Pianeze stain, which fact indicates that 
they are either lignified, cutinized, or suberized. In parenchyma tissues 
of this kind we should hardly expect to find lignin or cutin. When 
treated with concentrated potassium hydroxid, these walls gave the 
typical yellow reaction for suberin, which confirms the conclusion that 
they are of a corky nature. 

Taking into consideration the above-mentioned phenomena with other 
possibilities, it seems that a combined explanation might be offered for 
the resistance of flax to extensive invasion by F. lini. In the first place 



Dec. 10, 191 7 



Flaxwilt 587 



the protoplasm of the resistant plant may naturally contain a substance 
or substances injurious to the fungus. We know that the resistant plant 
differs from the susceptible plant in respect to its physiological nature. 
This difference might be due to some permanent chemical composition 
of the protoplasm as suggested above, or it might possibly be due to a 
hypersensitiveness of the protoplasm of the resistant plant which causes 
it to react much more readily than does the protoplasm of the suscepti- 
ble plant in producing the phenomena which cause resistance. The 
fungus seems to be less vigorous in the invaded cells of the resistant plant 
than in the invaded cells of the susceptible plant; in other words, it is 
less abundant in the cells of the resistant plant. It seems possible, 
therefore, that some toxic or other chemical substance is produced by 
the protoplasm of the host which has a deleterious effect on the fungus. 
The coarse, granular appearance and staining reaction of the protoplasm 
of the invaded cells indicate that considerable change has taken place. 
Apparently this change is accompanied by an injury to both the host 
cells and the fungus hyphae. Perhaps some substance is produced by 
the host protoplasm during the change which has an injurious effect on 
the fungus. When the hyphae of the fungus come in contact with the 
modified or corky walls of the cells they fail to penetrate, and further 
invasion is prevented. Possibly these thickened walls would not be 
sufficient within themselves to prevent invasion, but they serve as a 
barrier to the fungus after it has been weakened by protoplasmic reaction 
on the part of the invaded host cells. These phenomena seem to indi- 
cate that resistance is due either directly or indirectly to the chemical 
nature of the host protoplasm. Appel (1) believes resistance in plants 
to be of a chemical nature and makes the following statement : 

Efforts must be made to find the causes of immunity, and after solving this question 
to determine without infection the disease-resistant qualities in different varieties 
and individuals in order to be able to establish the desired resistance and at the same 
time eliminate undesirable qualities. 

Such a theory might at first seem entirely feasible; but, when the 
multiplicity of constitutional and environmental factors influencing the 
production of the resistant character is considered, it appears more im- 
probable that any such analysis will ever be satisfactorily made. Ward 
(23, 26) also speaks of the chemical nature of resistance. He (26, p. 21) 
says: 

Infection, and resistance to infection, depend on the power of the Fungus-proto- 
plasm to overcome the resistance of the cells of the host by means of enzymes or toxins; 
and, reciprocally, on that of the protoplasm of the cells of the host to form anti-bodies 
which destroy such enzymes or toxins, or to excrete chemotactic substances which 
repel or attack the Fungus-protoplasm. 

This theory might be offered as a partial explanation for the resistance 
of F. lini by flax plants. 



588 Journal of Agricultural Research vol. xi, no. u 

Finally, if we take into consideration the apparently weakened condi- 
tion of the fungus in resistant host cells, the change in the nature of the 
protoplasm of the invaded cells, the new cell division, and the formation 
of cork walls around the point of invasion, all of which seem to play a 
part in the prevention of further invasion by the fungus, and all of which 
are due more or less to the chemical reaction of the host protoplasm, it 
seems safe to conclude that the resistance of flax to F. lini is essentially 
of a chemical nature. 

INHERITANCE OF WILT RESISTANCE THROUGH HYBRIDIZATION 

In undertaking a study of the inheritance of wilt resistance though 
hybridization it was very necessary that highly resistant and susceptible 
strains of flax be secured and thoroughly tested on infected soil before 
making crosses. The object of this chapter is to deal with methods of 
procedure in the work and to give such results as have been obtained 
from crosses up to date. 

METHODS OF SOIL INOCULATION 

Before the progeny from crosses could be tested it was highly impor- 
tant that the soil on which the plants were to be grown should be thor- 
oughly infected with the wilt-producing organism, F. lint. The soil sent 
by Prof. Bolley from North Dakota was found by the preliminary tests 
to be satisfactory (PI. 44, B), but the quantity was not sufficient. An 
attempt was therefore made to inoculate soil with pure cultures of the 
organism. In order to try this out, a small flat of greenhouse soil was 
sterilized in an autoclave one half being planted to flax No. 3 (suscep- 
tible), and the other half to flax No. 4 (resistant). After planting, 
about half a dozen tube cultures of F. lini, which were fruiting abund- 
antly, were mixed thoroughly in a small pot of water and poured over 
the flat. Wilting of the susceptible plants did not begin until they were 
of considerable size, but they were completely killed in a short time 
after the disease started (PI. 44, C). A large bench of soil was then 
inoculated. Water suspensions of the organism from pure culture were 
poured over the soil and worked into the surface. Seeds of the suscep- 
tible strain of flax were planted in abundance in the soil. As more 
fruiting cultures of the organism were obtained, the inoculation was 
repeated. When the plants from this seed showed considerable wilt 
they were turned under the soil and more seed planted. Only three 
or four plantings of this kind were necessary with the pure-culture inocu- 
lations to put the soil in suitable condition for the growth of hybrid 
plants (PI. 46, A, B). It was also found that from i}4 to 2 inches of the 
North Dakota soil spread over Madison soil was sufficient to produce 
thorough wilting of susceptible plants (PI. 46, C, b). 



Dec. 10. 1917 Flaxwilt 5 8 9 

METHODS USED IN CROSSING AND SELFING PLANTS 

After thoroughly testing the different strains of flax on "sick soil" 
and selecting the most suitable ones for the work, crosses were made 
between plants of the resistant and susceptible strains. The first crosses 
were made with plants grown in the greenhouse on soil free from F. lint. 
These crosses were fairly successful. In the summer of 191 5, plants 
were grown in the field, and a large number of crosses were made between 
the different strains in a manner to be described later. These crosses 
were more successful than those made in the greenhouse. The best 
results were obtained from crosses made with the first few buds to appear 
on the plant. This is no doubt due to the fact that the plant is at its 
maximum sap content and highest state of activity at this stage and is 
more able to overcome any injury that might be done to the flower 
through the operation. After the plant becomes more mature, a lower 
percentage of the artificially pollinated flowers develops. The operation 
should be performed at about the time when the petals begin to show in 
the bud. At this stage the authers are not fully mature and are not 
so easily broken open. The petals can be removed very easily by catch- 
ing the tip of the bud with a small pair of forceps and pulling gently. 
Care should be taken not to catch too low on the bud, or the stigma 
may be broken or injured. Eyre and Smith (13) state that they removed 
the petals by a sudden jerk, removing the stamens at the same time. 
The writer was not able to do this with the varieties of flax used in this 
work, without injuring the stigma. 

After removing the petals the sepals can be pushed aside and the 
anthers removed by carefully pinching off the filaments with forceps, 
which should be sterilized by dipping them in 50 per cent alcohol before 
and after each operation to avoid contamination. After this operation 
the flowers are ready for pollination. Flowers pollinated a day after 
emasculation gave no better results than those pollinated immediately 
after the process. Pollination is easily accomplished by taking a flower 
which has just opened from the plant to be used as the male parent 
and brushing the anthers directly over the stigma of the emasculated 
flower. It is very easy to tell when the anthers are open by the mealy 
appearance of the pollen on the surface. Before opening they are smooth 
and white. After the flower has been pollinated it should be covered 
to keep out insects and other agents of contamination. Small glacine 
bags and waxed paper rolled on a fountain pen and tied at the ends 
were found to be quite satisfactory for protecting the flowers. After 
the capsules begin to develop it is better to remove the covering, espe- 
cially in the greenhouse, where there seems to be a smothering of parts 
covered in this way. In the field, where the rolled paper was used, the 
seed developed in good condition even though the paper was not re- 
moved. The bags, being tighter than the rolled paper, prevented aera- 



590 Journal of Agricultural Research vol. xi, no. » 

tion to a greater extent, and for this reason it was more desirable to 
remove them as soon as possible after the seed began to set. 

For selfing plants the paper bags were used very little. The whole 
plant was covered with cloth. The most satisfactory method was to 
use wire cylinders about 3 inches in diameter and about 12 inches long, 
made of screening and covered with slips of finely woven white cloth made 
to fit. These cloth slips should be considerably longer than the cylinders, 
so that they can be tied at both ends. A piece of stout wire is pushed 
into the ground beside the plant, so that it extends a few inches above 
the top of the plant. The cylinder is then placed over the wire and the 
plant, and the cloth is brought together at the upper end and tied tightly 
around the large wire above the top of the plant. The lower end of the 
cloth is then tied around the wire and the plant above ground. The 
fruiting part of the plant is thus protected within the cylinder, where it 
produces seed in a fairly normal manner. These cylinders should be 
removed as soon as the flowering period is over and the fruit has set. 

METHODS USED IN GROWING THE PROGENY FROM CROSSES 

Plants of the first and second generations which were to be tested for 
resistance were grown in flats and on benches of North Dakota "flax- 
sick soil" and Madison soil which was inoculated as previously stated. 
These experiments were conducted in the greenhouse throughout the 
year. There is a slight variation in temperature in the greenhouse 
with change of seasons, a condition that can not be prevented. It was 
shown by temperature studies {23) that a difference of a few degrees might 
greatly influence the rate and amount of attack of flax by F. lini. By 
comparing the controls grown in winter and summer this difference will 
be observable. The high summer temperatures increased the severity of 
the wilt, even a few plants of the resistant strain wilting. 

The soil was well pulverized and the seed planted about 1 inch apart 
in rows about 4 inches apart. This made it possible to grow a large 
number of plants in a comparatively small area. In every case rows of 
both parent strains were planted in every flat or bench in sufficient 
numbers to serve as controls, and in some experiments selfed seed from 
the parent plants of the crossed seed wore planted as controls. The 
number of seeds planted was recorded in order to ascertain the percentage 
of germination. In the summer, when the greenhouse temperature 
ran high, the percentage of germination was low. In some cases practi- 
cally none of the seeds germinated. This perhaps was not due to tem- 
perature alone but to the increased activity of other physical and 
biological agents in the soil. As soon as the seed germinated and the 
seedlings appeared above the ground, they were counted and recorded 
as plants. Any plant that made its appearance above the ground was 
counted, even though it died in this stage from wilt. The results are 



Dec. io. i 9 i 7 Flaxwilt 591 

given with this stage of the plant as a starting point. It would be 
impossible to determine in every case the exact reason for the failure 
of the seed to germinate; and as some seeds of both strains did not 
germinate, it was not thought wise to attribute any of the failure directly 
to F. lini, although it is quite likely that this organism was partly respon- 
sible. If there was any doubt as to the cause of the wilting of the seed- 
ling after it appeared above the ground, it was determined by isolation 
methods in the laboratory. After the number of plants was recorded 
they were kept under almost daily observation. Notes were taken 
every week where possible and the number of healthy, wilted, and dead 
plants recorded. Any plant which showed undoubted wilt symptoms 
was recorded as wilted. This method of note-taking made it possible 
to compare the rate of wilting of the hybrid plants with that of the 
susceptible strain. As time and space were very limited it was found 
to be undesirable to grow all plants to complete maturity. It was then 
necessary to select some stage in the development of the plant as the 
end point for observation and note-taking. At this stage all plants 
which were not to be kept for seed could be removed and other seed 
planted. The flowering stage was selected as being the most favorable 
to cease note-taking, for at this stage the plant has reached its maximum 
activity and very little noticeable infection takes place after this time. 
Part of the hybrid seed was grown in clean soil in a different green- 
house where there was no chance for infection by F. lini. These plants 
were self-fertilized as previously described in order to obtain seed for 
the next generation. By this means seed was obtained from plants 
that might have been destroyed by wilt if they had been grown in 
infected soil. 

RESULTS OBTAINED FROM THE CROSSES 

The parent strains, as previously stated, were thoroughly tested on 
"flax-sick soil" before the crosses were made and were found to be 
uniformly resistant or susceptible, as the case might be. However, 
some of the plants of the resistant strain wilted in later experiments. 
Results obtained from the progeny of crosses show that there is a great 
difference in individuality among plants of any strain with respect to 
its resistance to wilt. This difference was shown very strongly in the 
two generations grown, the results being so widely different from indi- 
vidual crosses that it will be necessary to discuss the different crosses 
or groups of crosses separately. The first generation from certain 
crosses proved to be entirely, or almost entirely, resistant to wilt. Others 
were intermediate with respect to resistance, while still others were 
entirely susceptible. In cases where there was entire or partial sus- 
ceptibility a difference in the time and rate of infection was noticed 
as compared with the time and rate of infection of the common, suscepti- 
ble flax. In some experiments the common flax would be almost entirely 



592 Journal of Agricultural Research vol. xi. no. h 

dead before the first-generation plants began wilting (PI. 45, A, B). 
Later all of these I<\ plants would perhaps succumb to the disease. 
In some of the experiments there was segregation in the first-generation 
offspring, part of the plants wilting, while others were as resistant as the 
resistant parent. In this case there was an intermediate condition as 
to the time of infection of the F x plants also. This fact was apparently 
not due to excessive vigor of the plants, since vigor seems to play no part 
in the resistance to wilt by the flax plant. The plants of common flax 
seem to be even more vigorous than resistant plants in cases, but suc- 
cumb very readily to the attacks of the fungus. 

No such simple ratios were obtained from these flax crosses as Biffen 
(2, 3) reported from his wheat crosses. Biffen was studying the nature 
of the inheritance of resistance by wheat to yellow-rust. In his first set 
of experiments (2) he crossed Rivet wheat, which is resistant to yellow- 
rust, with Michigan Bronze, which he states is probably more suscep- 
tible to yellow-rust than any other wheat in existence. The first genera- 
tion from this cross was entirely susceptible to attacks by the rust. No 
seed was obtained for a sesond generation, owing to the severity of the 
attack. Red King, a very susceptible variety, was crossed on Rivet, 
and the first generation was susceptible in this case also. A second gen- 
eration from these plants gave practically a i-to-3 ratio, which Biffen 
interpreted as indicating that resistance and susceptibility are unit char- 
acters, the latter being dominant to the former. Biffen fails to state just 
where he drew the line between resistance and susceptibility, a very 
important point. He says the plants which he placed in the resistant 
group were "relatively" or "almost" free from rust. He also states 
that the third generation gave results which confirmed those of the pre- 
ceding generations, but, that statistics for this last generation were not 
altogether satisfactory, owing partly to the limited amount of grain har- 
vested for the trial and partly to the unfavorable conditions at the time 
of sowing. 

In 1907 Biffen (j) published on a piece of work which was much more 
thorough apparently than his first work. He reported in this case, as 
before, that the first generation of wheat plants from a cross between 
entirely immune and susceptible strains was entirely susceptible, and 
that the second generation gave approximately a ratio of one resistant to 
three susceptible plants. In these experiments every plant that showed 
symptoms of rust was considered as suscept ible. He also states that there 
was a gradation between entirely resistant and entirely susceptible 
plants. 

Stuckey (22), working with tomatoes at the Georgia Station, states that 
by crossing the Red Cherry, which is resistant to blossom endrot, with 
the Greater Baltimore, a large commercial type which is susceptible to the 
disease, both the first and second generations from the cross were resist- 
ant to blossom endrot and that there was no segregation in the second 



Dec. 10, 1917 



Flaxwilt 593 



generation according to Mendelian laws. Unless there is some possi- 
bility that these fruits were not subjected to uniformly favorable condi- 
tions for blossom endrot, this is apparently an unusual case of inheritance. 

Orton (20, p. 463) says, in writing on the resistance of farm crops to 
disease : 

When a disease-resistant variety is crossed with a nonresistant variety, the resulting 
offspring inherit resistance to a limited and varying extent. 

The writer found that in crosses between resistant and susceptible 
flax plants there was considerable difference in the results obtained 
from the different progenies. The results seemed to depend largely on 
the individual plants crossed. One of the most promising crosses made 
was one between the resistant strain No. 4 and the most susceptible 
strain, No. 3. The resistant plant was used as female parent, and the 
progeny was designated as 4D20, "4" referring to the strain, "D" to the 
plot where the female parent was grown, and "20" to the number of 
the female plant. With these data on the female parent it was easy to 
refer to the original records of the cross for the strain of the male parent. 
This system of recording was used throughout the work. From crosses 
between the 2 plants mentioned above 5 capsuls were obtained, and 
26 first-generation plants were grown on soil thoroughly infected with 
F. lini (PI. 45, D, b). These plants were distributed among three 
flats, one containing 8 and the two others containing 9 plants each. 
The plants in one fiat were growing at a lower temperature than those in 
the two other flats, the temperature of the former ranging from 14 to 
1 9 C, while that of the latter ranged from 18 to 21 . None of these 
F x plants, however, were infected, although the controls of susceptible 
flax No. 3 wilted completely (23). They were grown to maturity and 
seed was obtained for a second generation by selfing. There was a segre- 
gation in the second generation into resistant and nonresistant plants. 
Table III gives the results obtained from the first and second generations 
and their controls. In this case selfed seed from the parent strains were 
grown as controls. In order to show the comparison between hybrid 
plants and plants of the susceptible strain, the number of plants wilted 
at the end of three weeks and the number killed by wilt at the end of the 
experiment were recorded. 

The second-generation plants, as indicated by the controls given in the 
table, were grown under somewhat more severe conditions than the first 
generation, as is shown by the fact that a number of the plants of the 
resistant parent strain were killed or infected by wilt. The F^ plants 
were grown in pure North Dakota soil, while the plants of the second 
generation were grown in artificially infected soil. Bolley (9) says that 
a change in the type of soil may cause a weakening of the resistant 
character. There was also a difference of temperature under which 
these F x and F 2 plants were grown, which may have played some part. 
The F 2 plants were grown in the summer and autumn, while the F x plants 



594 



Journal of Agricultural Research 



Vol. XI, No. ii 



were grown in winter and early spring, when the temperature was con- 
siderably lower. A slight rise in temperature serves to accelerate the 
growth of the fungus, and the disease becomes more severe. 

Table III. — Resistance to flaxwilt obtained from the progeny of a cross between resistant 
flax No. 4 9 . and susceptible No. 3<$ a 



Parent strain. 



Date of planting. 



Num- 
ber of 
plants 
grown. 



Ratio at end of 
three weeks. 



Wilted. 



Not 
wilted. 



Ratio at end of 
experiment. 



Wilted. 



Resist- 
ant. 



Num- 
ber of 
plants 
killed 
by 
wilt. 



4D20CF!) 

Resistant No. 4. . 
Susceptible No. 3. 

F 2 generation: 

4D20-1 

4D20-2 

4D20-3 

4D20-4 

4D20-5 

4D20-6 

4D20-8 



Feb. 



1916. 



.do. 
.do. 



26 
39 

52 



o 
o 

34 



26 
39 



26 
39 



5 2 



o 

52 



July 22 . 
Sept. 23 

do.. 

do.. 

do.. 

do. . 

do.. 



68 
98 

"5 
28 

101 

50 
70 



Total 



53° 



Resistant No. 4 . . 
Susceptible No. 3. 



Sept. 23 
do.. 



56 

82 



37 
42 
40 
9 
59 
29 
54 



3 1 
56 
75 
19 
42 
21 
16 



45 
62 
08 
22 
76 

34 
61 



23 
36 

47 
6 

25 
16 



40 
S3 
47 
19 
64 
3° 
53 



270 260 



368 162 



306 



6 
76 



5° 
6 



10 

82 



46 
o 



6 

82 



a 9=female; d" =male. 

Since the first generation from this particular cross was entirely 
resistant, it was hoped that some reasonable explanation might be given 
for the results obtained in the second generation, although it was appar- 
ent at once that they could not be explained on a unit-factor basis. As 
the number of susceptible F 2 plants was very large, it was thought that 
they might be explained by Little's (16) hypothesis, which is an expla- 
nation of cases that appear to be a reversal of dominance. The indi- 
viduals showing the character in question decrease in number in the 
F 2 generation, as there is an increase in factors which produce the par- 
ticular character. The general principle is that with the addition of 
each factor involved the number of F 2 individuals possessing the char- 
acter in question is multiplied by 3, while the total number of F 2 indi- 
viduals is multiplied by 4. The difference between the number of indi- 
viduals with the character and those lacking it grows progressively 
greater with each factor added. With the flax cross under considera- 
tion, the first generation was entirely resistant to wilt. In the second 
generation, from a total of 530 plants 162 were resistant and 368 suscep- 
tible. These figures approach very closely the expectation, if four factors 
are concerned in producing resistance. The actual expectation would be 
81 resistant to 175 susceptible, which is a ratio of 1 to 2.16. The actual 
proportion obtained was 81 resistant to 184 susceptible, which is a ratio 



Dec. 10, 1917 



Flaxwilt 



595 



of 1 to 2.27. This ratio is fairly close to the expectation. The number 
of susceptible plants ran rather high, although this was to be expected, 
since the F 2 plants were grown under slightly abnormal conditions, as we 
have already seen. Some of these plants showed only slight signs of 
infection and would no doubt have resisted entirely had they been under 
less severe conditions. In this case the discrepancy is in the direction 
expected, the number of susceptible plants running high. Since this is 
the only cross that gave definite ratios, too much emphasis should not be 
placed on these results until further experimental evidence is obtained. 
It is furthermore very desirable that experiments of this kind be con- 
ducted in environmental conditions which are kept fairly constant. 

Table IV. — Resistance to flaxwilt obtained with reciprocal crosses of resistant flax No. 4 
with susceptible No. 3 and their controls 



Parent strain. 



4Ei(F 1 ). 

6E1 

4E1 (self) , 
6E1 (self) 



F 2 generation(6Ei ? and4Ei<? 
6E1-1 



6E1-2 . 
6E1-3 ■ 
6E1-4. 
6E1-5. 
6E1-6 . 



Total. 



Controls, F 2 generation: 
Resistant No. 4. . . 
Susceptible No. 6. . 
Susceptible No. 3. . 



F 2 generation (4E1 9 , 6E1 S )'■ 
4E1-1 



4E1- 
4E1-3 ■ 
4E1-4 • 
4E1-5 • 
4E1-6. 



Total. 



Date of 
planting. 



I9I5- 

Oct. 15 

..do 

..do 

..do 



1916. 

June 27 
...do.... 

..do.... 

..do.... 
...do.... 
...do.... 



.do. 
.do. 
.do. 



.do. 
.do. 
.do. 
.do. 
.do. 
.do. 



Num- 
ber of 
plants 
grown. 



76 

133 

125 

104 

66 

34 



538 



84 

S 

24 



93 
74 

36 
86 
5& 



429 



Ratio at end of 
three weeks. 



Wilted. 



56 
IOO 

56 

49 
22 

15 



24 



61 

5 2 
33 
63 
3i 
55 



295 



Not 
wilted. 



20 
33 
79 
55 
44 
19 



240 



72 

o 

10 



32 

22 

3 

2 3 
25 
19 



134 



Ratio at end of 
experiment. 



Wilted. 



67 

128 

123 

101 

66 

34 



519 



3° 



24 



92 

73 
30 

86 

56 

80 



423 



Resist- 
ant. 



17 



19 



54 



Num- 
ber of 
plants 
killed 
by 
wilt. 



9 
9 
o 

17 



57 
"5 
118 

97 
65 
33 



485 



24 



68 
36 
84 

55 
78 



405 



Another cross which was followed up with great care was a cross of 
resistant flax No. 4, with susceptible No. 6. Reciprocal crosses were 
made, and selfed seed was obtained from both parent plants. The 
resistant parent is designated as 4E1 and the susceptible parent as 
6E1. The first generation (PI. 45, A, b, c) proved to be entirely suscept- 



596 



Journal of Agricultural Research 



Vol. XI, No. xi 



ible, although it might be said that there was an intermediate condition, 
owing to the fact that there was considerable difference in the time 
of wilting of the first-generation plants as compared with plants grown 
from selfed seed from the susceptible parent. The second generation 
was almost entirely susceptible. These plants, however, were grown 
under more severe conditions than were the plants of the first genera- 
tion. The severeness of these conditions can be seen from results of 
the resistant control given in Table IV. 

In this case, as is shown by the table, the second-generation plants 
were grown under fairly severe conditions. Thirty of the eighty-four 
plants of the resistant control were infected. The second generation 
was grown on artificially infected soil. Considering the behavior of the 
first generation and the severe conditions under which the second gen- 
eration was grown, together with the results obtained, it seems impos- 
sible at present to give any direct explanation on a Mendelian basis. 

In order to see what the result would be if the seed from the crosses 
on a number of plants were mixed, seed from five different plants were 
thrown together and planted. In this case sight was lost of the indi- 
viduality of parent plants, due to the mixing; consequently, no definite 
ratios could be expected as individuals behave very differently. Table 
V gives the results obtained from mixed progeny. 

Table V. — Resistance to flaxwilt of the mixed progeny of crosses between resistant flax 
No. 4<S and susceptible No. 3 9 





Date of 
planting. 


Num- 
ber of 
plants 
grown. 


Ratio at end of 
three weeks. 


Ratio at end of 
experiment. 


Num- 
ber of 
plants 




Wilted. 


Not 
wilted. 


Wilted. 


Resist- 
ant. 


killed 

by 

wilt. 


^E-Mix. (F,) 


I9I5- 

Oct. 2 

... do ... . 


1 53 
20 
46 


53 


43 


IOO 

20 

3 


106 

O 

46 


47 

20 




76 
O 


Resistant No. 4 




... do ... . 


46 




1916. 
Apr. 22 
... do ... . 


F 2 generation: 

3E-M1X-1 


89 
81 
88 


4i 
63 

55 


48 
18 
33 


49 
78 
84 


40 
3 
4 


46 


3E-Mix-2 


72 
67 


^E-Mix-3 


... do 






Total 


258 


159 


99 


211 


47 


185 




Apr. 22 
... do ... . 


Resistant No. 4 


45 
43 


1 
33 


44 
10 


a 3 
43 


42 



O 


Susceptible No. 3 


43 







a Slight. 

As is shown by the controls in Table V, these plants were grown under 
very good conditions for the best test. The resistant parent strains 
stood up almost perfectly, while every plant of the susceptible parent 
strain died out completely. Since there are all gradations between an 
entirely resistant and an entirely susceptible first generation from indi- 
vidual crosses, we would expect the intermediate condition when seed 



Dec. 10, 191 7 



Flaxwilt 



597 



from a number of individuals are mixed. This was what actually hap- 
pened. In the second generation there was great variation in the prog- 
eny from individuals of this first generation (PI. 46, B, c, h). This is 
what might be expected, since the chances are that each group of second- 
generation plants came from a different cross. This method of growing 
the progeny was found to be very unsatisfactory as it gave no direct line 
on individual crosses which have been found to behave so differently. 

Table VI. — Resistance to flaxwilt of the progeny from crosses between resistant and sus- 
ceptible strains of flax in which about an equal number of resistant and susceptible 
plants occurred in the first generation 

RESISTANT NO. 4 9 X SUSCEPTIBLE NO. 6 c? 





Date of 
planting. 


Num- 
ber of 
plants 
grown. 


Ratio at end of 
three weeks. 


Ratio at end of 
experiment. 


Num- 
ber of 
plants 




Wilted. 


Not 
wilted. 


Wilted. 


Resist- 
ant. 


killed 

by 

wilt. 


4E2 (F,) 


1916 
Apr. 25 
...do 


6 
6 
9 


I 
O 
9 


5 
6 



3 
O 

9 


3 
6 



I 


Resistant No. 4 


O 


Susceptible No. 6 


. ..do 


9 




Sept. 23 
...do 


F 2 generation: 

4E2-1 


26 
37 


8 

17 


18 
20 


IS 
24 


11 
13 


14 


4E2-2 


21 








Total 


63 


25 


38 


39 


24 


35 




Sept. 23 
...do 


Resistant No. 4 


26 

24 


4 
14 


22 
10 


7 
24 


19 



5 


Susceptible No. 3 


24 









RESISTANT NO. 4 9 X 


SUSCEPTIBLE 


NO. 3 


3 






4D3 (Fj) 


1916 
Feb. 2 
...do 


18 

17 
16 





2 


18 
17 
14 


10 



16 


8 

17 



2 


Resistant No. 4 





Susceptible No. 3 


...do 


11 




July 21 
...do 




F 2 generation: 

4D3-1 


74 

63 

i°5 


47 
43 
69 


27 
20 

36 


73 

61 

103 


1 
2 
2 


53 


4D 7—2 


46 


D^-3 


...do 


82 








Total 


242 


159 


83 


237 


5 


181 




July 21 
...do 




Resistant No. 4 


37 
26 


6 

25 


31 

I 


14 
26 


23 



8 




26 



RESISTANT NO. 4 d AND SUSCEPTIBLE NO. 3 $ 



3E22(F!) 

Resistant No. 4 

Susceptible No. 3 

F 2 generation: 

3E22-2 

Resistant No. 4 . . 

Susceptible No. 3. 



1916 
July 2 1 

..do 

..do 



Sept. 23 

..do 

..do 



9 


1 


8 


4 


5 


15 





15 





15 


18 


16 


2 


18 





24 


20 


4 


2 3 


1 


26 


4 


22 


7 


19 


24 


14 


10 


24 






3 

o 
18 

22 

5 

24 



598 



Journal of Agricultural Research 



Vol. XI, No. ii 



A number of cases were discovered in which the first generation split 
up into almost equal numbers of resistant and susceptible plants, but 
there seemed to be no definite order of segregation in the second genera- 
tion in cases of this kind. The severeness of conditions under which 
these plants were grown can be judged from the controls in each case. 
Table VI gives results of crosses of this kind. 

Another grouping of tables has been made to show the results of 
crosses where there was a larger number of resistant than susceptible 
plants in the first generation. There seems to be but little relation 
between the behavior of the first and second generations from these 
crosses. Table VII gives the results of crosses of this kind. 

Table VII. — Resistance to flaxwilt of crosses between resistant and susceptible strains of 
flax in which a greater number of resistant than susceptible plants occurred in the first 
generation 

RESISTANT NO. 4 9 X SUSCEPTIBLE NO. 3 c? 



Parent strain. 



4Di(F0 

Resistant No. 4. . 
Susceptible No. 3. 

F 2 generation : 

4D1-2 

4D1-3 

4D1-4 

4D1-5 

4D1-6 

4D1-8 



Total 



Resistant No. 4 . . 
Susceptible No. 3. 



Date of 
planting. 



1916. 

Mar. 29 

..do 

..do 



July 22 
..do.... 
..do.... 
..do.... 

..do 

..do 



.do. 

.do. 
.do. 



Num- 
ber of 
plants 
grown. 



21 
95 

8 S 



47 
49 
4i 
32 

47 
49 



265 



Ratio at end of 
three weeks. 



Wilted. 



4 

5 
80 



32 

41 

35 
24 

35 
45 



55 
47 



12 
45 



Not 
wilted. 



17 

90 

5 



53 



43 



Ratio at end of 
experiment. 



Wilted. 



4 
8 

85 



42 
47 
37 
28 

43 

47 



244 



37 
47 



Resist- 
ant. 



17 

87 



Num- 
ber of 
plants 
killed 
by 
wilt. 



6 

85 



36 
42 

34 
24 

36 

42 



214 



*5 
46 



RESISTANT NO. 4 $ X SUSCEPTIBLE NO. 5 9 



sEiCFX 

Resistant No. 4. . 
Susceptible No. 5. 

F 2 generation : 

5B1-1 

5E1-2 

5E1-3 

5E1-4 

5E1-5 



Total 



Resistant No. 4 . . 
Susceptible No. 5. 



Feb. 
. .do. 
..do. 



Sept. 23 

..do 

..do 

..do 

..do 



.do. 

.do. 
.do. 



16 



78 
8 
18 
24 
20 



138 



20 



56 



14 
7 



87 



14 



22 
6 
10 
10 
13 



5i 



22 
o . 



16 



67 

4 
18 
21 
13 



123 



25 



19 



o 

o 

16 



5* 
4 
15 
18 
10 



98 



Dec. 10, 1917 



Flaxwilt 



599 



Table VII. — Resistance to flaxwilt of crosses between resistant and susceptible strains of 
flax in which a greater number of resistant than susceptible plants occurred in the first 
generation — Continued 

RESISTANT NO. 4 c? X SUSCEPTIBLE NO. 5 9 



Parent strain. 



5E22(F,) 

Resistant No. 4 

Susceptible No. 5 

F 2 generation : 

5E22-1 

Resistant No. 4 . . 

Susceptible No. 5. 



Date of 
planting. 



1916. 

Apr. 26 
..do.... 
..do.... 



Sept. 23 

..do 

..do.... 



Num- 
ber of 
plants 
grown. 



14 
iS 
16 



S3 

26 



Ratio at end of Ratio at end of 
three weeks. experiment. 



Wilted. 



o 
o 

16 

21 

4 
2 



wfft°ed. lilted. 



14 

18 

o 

3 2 

22 

o 



5 

o 
16 

40 

7 

2 



Resist- 
ant. 



Num- 
ber of 
plants 
killed 
by 
wilt. 



I 

o 

16 

32 
5 



RESISTANT NO. 4 $ X SUSCEPTIBLE NO. 3 ° 



3Ei 4 (F,) 

Resistant No. 4. . . . 
Susceptible No. 3 . . . 
F 2 generation : 

3E14-1 

Resistant No. 4. 



Feb. 
..do. 
..do. 



July 
. .do. 



Susceptible No. 3 1 . . .do. 



9 





9 


2 


7 


14 





14 





14 


21 


7 


14 


21 





80 


62 


18 


79 


1 


26 


4 


22 


7 


19 


24 


14 


10 


24 






67 

5 
24 



RESISTANT NO. 4 $X SUSCEPTIBLE NO. 5 <? 



dEnCF.) 


Oct. 18 
...do 


5 
79 
5 1 




3 

36 


5 
76 

J 5 


2 
6 

5 1 


3 

73 








3 




...do 


5° 




...do 




F 2 generation : 


202 

128 
141 


55 
63 
3° 


147 

65 

in 


137 
116 

IOI 


65 
12 

40 


86 




...do 


86 




...do 


56 








Total 


47i 


148 


323 


354 


117 


228 









RESISTANT NO. 4 9 X SUSCEPTTBLE NO. 5 6* 



4 E7(F,) 


Oct. 18 
...do 


5 
79 
5i 




3 
36 


5 
76 

15 


2 
6 

5 1 


3 

73 









3 




...do 


50 




...do 




F 2 generation : 

4E7-1 


44 
52 


10 

7 


34 
45 


18 
16 


26 
36 


10 




...do 


11 










96 


17 


79 


34 


62 


2! 









a«Controls for F2 same as for Fi above. 



6oo 



Journal of Agricultural Research 



Vol. XI, No. ii 



Table VIII shows the results of crosses in which the first generation 
was entirely or almost entirely susceptible. The second generation in 
these cases behaved about as did the second generation from crosses 
which have been given above. There seems to be no common explana- 
tion for segregation in this case, which, of course, is to be expected when 
the first generation segregates irregularly, as was true with these crosses. 

Table VIII. — Resistance to flax wilt of crosses between resistant and susceptible strains of 
flax in which the first generation plants were entirely or almost entirely susceptible 

RESISTANT NO. 4 9 X SUSCEPTIBLE NO. 5 $ 



Parent strain. 



Date of 
planting. 



Num- 
ber of 
plants 
grown. 



Ratio at end of 
three weeks. 



Wilted. 



Not 
wilted. 



Ratio at end of 
experiment. 



Wilted. 



Resist- 
ant. 



Num- 
ber of 
plants 
killed 
by 
wilt. 



4Ei 3 (F 1 ) 

Resistant No. 4 

Susceptible No. 5 

F 2 generation: 

4E13-1 

Resistant No. 4. .. 

Susceptible No. 5. 



1016. 

Feb. 2 

..do... 

...do... 



Sept. 23 
...do.... 
...do.... 



14 



127 
26 



82 
4 



45 



126 

7 



3 

o 

12 

125 
5 



RESISTANT NO. 4 ° X SUSCEPTIBLE NO. 5 6* 



4El2(F 1 ) 

Resistant No. 4 

Susceptible No. 5 

F 2 generation: 4E12-1 



Oct. 18 
..do..., 
..do... 
..do... 



79 

123 



5 

3 

36 

62 



6 

76 

61 



51 
106 



73 
o 

17 



8 

3 

5° 

83 



a Control same as for Fi. 

A large number of first-generation plants have been grown from dif- 
ferent crosses from which no second-generation plants have been grown. 
The first generations from these crosses vary from those which are 
entirely resistant to those which are entirely susceptible, as have been 
given in the previous tables. Table IX will show these results. 

DISCUSSION OF RESULTS 

The inspection of Tables III to IX reveals no common explanation for 
the results obtained. Except for one case (Table III), no definite ratios 
could be detected. However, there are a number of possible reasons 
for these results: (1) The plants were grown under abnormal green- 
house conditions. (2) The temperature was too high in some cases, as 
previously emphasized. (3) A different type of soil was used for some 
of the tests, which, according to Bolley (o), is likely to cause a breaking 
down of the resistant character. (4) There is a great variation among 
individual plants within a strain. 



Dec. 10, 1917 



Flaxwilt 



601 



Table IX. — Resistance to flax wilt of the first generation from individual crosses between 
resistant and susceptible strains of flax 



Parent strain. 



4D3QCF!) 

4D8(F 1 ) 

Resistant No. 4 

Controls: 

Susceptible No. 3 . 

4D21 

4D38 

4D37 

4D29 

4D26 

4l>3 2 

4D-mix 

Resistant No. 4. . . 
Susceptible No. 3. 

4D36 

4D31 

4D23. 

4D-mix 

Resistant No. 4. .. 
Susceptible No. 3 . 

6E2 

4E8 

5E3 

4E6 

Resistant No. 4 . . 
Susceptible No. 5 . 



Date of 
planting. 



1916. 
Feb. 2 
..do... 
..do... 



..do.... 
Mar. 29 

..do... 

..do... 

..do... 

..do... 

..do... 

..do... 

..do... 

..do... 

May 13 
...do... 
...do... 
...do... 
...do... 
...do... 

Apr. 25 
...do... 
...do... 
...do... 
...do... 
...do... 



Num- 
ber of 
plants 
grown. 



Ratio at end of 
three weeks. 



Wilted. 



18 
36 
iS 



32 

21 
16 
17 

88 

95 

85 
16 



40 
63 

75 
6 
8 

5 

10 



o 
o 
o 

5 
1 
6 

17 
16 

9 
9 

5- 
S 

So 
6 
9 

15 

32 
7 

74 
1 

3 
4 
7 
o 

4 



Not 
wilted. 



Wilted. 



36 



13 

5 

16 

13 
5 
7 
8 

36 
90 

5 

10 
11 

5 
8 

56 

1 

5 
5 

1 

3 
10 

1 



Ratio at end of Num- 
experiment. j-, er f 
plants 
killed 
Resist- by 
ant. wilt. 



17 
6 

7 
32 
21 
16 

17 

82 

8 

85 

6 

10 

iS 
36 
26 

75 
3 
4 
5 
9 
1 

5 



15 



6 

87 

o 

10 

10 

5 
4 

47 
o 

3 



o 

4 
o 

13 

1 

3 
26 
20 
12 
J 7 

60 

6 

85 
6 
6 

11 

26 
4 

75 
1 
2 
3 
4 
o 

5 



However, it seems that there is a possible explanation for the results 
obtained. The resistant strain of flax has been bred by selection to 
resist wilt under certain environmental conditions which we might call 
normal. As was pointed out in Table III, resistance is probably due to 
a number of factors. Then it might be assumed that under the normal 
environment certain factors, A, B, C, for example, are required to pro- 
duce resistance. These factors, which may be of equal or unequal value 
in producing resistance, are possibly homozygous, as is evidenced by the 
fact that segregation is exceptional under what is termed a normal 
environment. If, however, the environment is made more severe, there 
is a certain amount of segregation among plants of the resistant strain, 
which was found to be true with plants growing in the greenhouse in the 
summer. Under these more severe conditions one or more additional 
factors, D, B, etc., might be supposed to be required to cause the plant 
to resist. Since the strain has not been selected to resist under these 
severe conditions of environment, no attempt has been made to make 
these factors homozygous; therefore they may be absent entirely, or 
may occur singly or in combination, and may be either homozygous or 



602 Journal of Agricultural Research vol. xi. no. « 

heterozygous. Then, a certain amount of segregation would be expected 
when the plants are subjected to these conditions. Segregation would 
likewise be expected to take place in the first-generation offspring from 
crosses of these plants with plants of the susceptible strain, provided 
the plants are grown in the abnormal environment. Differences of the 
individual plants of the susceptible strain with regard to the resistant 
character might account for considerable variation in the amount of 
segregation in the first generation from such crosses. A difference of 
this kind has been the basis for the selection of resistant strains. It is 
possible that some of these plants have one or more of the factors for 
resistance, which may be either homozygous or heterozygous, but do 
not have a sufficient number of them to cause the plant to resist where 
the disease develops normally. In crossing susceptible plants of this 
kind with plants of the resistant strain various factor combinations 
would be obtained, some of which would produce resistance in the first 
generation under the severe conditions while others would not. Results 
obtained in these experiments could easily be correlated with a theory 
of this kind. 

The perfecting of methods rather than the development of any defi- 
nite proof of laws governing the inheritance of wilt resistance in flax 
has been the main accomplishment in this work. Therefore a few help- 
ful suggestions might be offered by way of conclusion of this chapter. 

(i) The resistant plant should be tested under severe disease con- 
ditions before making the cross. (2) Reciprocal crosses should be 
made in each case, and selfed seed should be obtained from both parent 
plants, this to be planted along with the crossed seed. (3) First-gen- 
eration plants should be grown in disease-free soil to obtain seed for a 
second generation. (4) First and second-generation plants which are 
to be tested should be grown under similar conditions with selfed seed 
from the parents planted as controls. (5) The experiments should be 
conducted under uniform environmental conditions in order to obtain 

conclusive results. 

CONCLUSIONS 

(1) The flax plant is most suitable for a study of the nature and in- 
heritance of wilt resistance, since it grows very well in the greenhouse, 
has a short growing season, is easily crossed, resistant and susceptible 
strains are available, and conditions for infection can be produced 
with certainty. 

(2) Fusarium lini penetrates the flax plant through root hairs, young 
epidermal cells, stomata of seedlings, and perhaps through wounds. 

(3) Fusarium conglutinans is able to penetrate the root hairs of 
young cabbage seedlings when the seedlings are grown in tube cultures. 

(4) F. lini invades the various tissues of the susceptible flax plant 
causing the disease known as flaxwilt. No considerable clogging of 



Dec. 10, 1917 Flaxwilt 603 

vessels can be seen. Wilting may be due to the combined action of 
several factors: (a) Destruction of the young active root system by the 
fungus, which cuts off a part of the food and water supply of the plant, 
(b) Use of the food and water supply of the plant by the fungus, (c) 
More vigorous growth of the fungus and increased transpiration of the 
host plant due to a rise in the temperature, (d) The possible pro- 
duction of toxins by the fungus which injure the host protoplasm. 

(5) F. lini penetrates the resistant flax plant and stimulates division 
and cork wall formation in cells adjacent to those attacked, but is not 
able to invade the tissues to any considerable extent owing to a number 
of possible reasons: (a) The permanent chemical composition of the re- 
sistant plant may be of such nature as to be injurious to the fungus, 
(b) The protoplasm of the resistant plant may be more highly sensi- 
tive than that of the susceptible plant, thus reacting more readily in 
the production of those phenomena which cause wilt resistance. (3) 
The stimulation to new cell division and the laying down of cork walls 
which seem to serve as a barrier to further invasion by the already 
weakened hyphae. 

(6) Wilt resistance in flax is an inheritable character which is appar- 
ently determined by multiple factors. 

(7) There is a great difference in the individuality of plants of a strain 
with respect to the resistant character, as shown by their offspring. 
The first generation from some crosses is entirely resistant, from some 
intermediate, and from others entirely susceptible. 

(8) The degree of resistance shown by a strain of flax depends to a 
considerable extent on the environmental conditions under which the 
plants are grown. A strain which was bred to resist under certain con- 
ditions may break down under a more severe environment. Plants of 
North Dakota Resistant No. 114, the best strain employed in this work, 
was not entirely resistant with the high summer temperatures in the 
greenhouse. 

(9) All parent strains to be used in crossing should be thoroughly 
tested on infected soil under favorable disease conditions before making 
the crosses. The resistant parent to be used in the cross should be grown 
on infected soil. 

(10) Hydridization experiments should be conducted under uniform 
environmental conditions in order to obtain conclusive results. The 
fact that such varied results were obtained in this work is probably 
due to the different environments under which the plants were grown. 



604 Journal of Agricultural Research vol. xi, no. w 

LITERATURE CITED 
(i) Appel, Otto. 

1915. disease resistance in plants. In Science, n. s., v. 41, no. 1065, 
p. 773-782. 
(2) BlFFEN, R. H. 

1905. MENDEL 'S LAWS OF INHERITANCE AND WHEAT BREEDING. In Jour. 

Agr. Sci., v. 1, pt. 1, p. 4-48, 2 pi. 

(3) 

1907. STUDIES IN THE INHERITANCE OF DISEASE RESISTANCE. In Jour. Agr. 
Sci., v. 2, pt. 2, p. 109-128. 
(4) BOLLEY, H. L. 

1901 . flax WILT AND FLAX-SICK SOIL. N. Dak. Agr. Exp. Sta. Bui. 50, p. 27-58, 

16 fig. 



(5) 
(6) 

(7) 
(8) 

(9) 

(10) 
(II) 



1906. flax culture. N. Dak. Agr. Exp. Sta. Bui. 71, p. 141-216, 22 pi. 



1907. PLANS FOR PROCURING DISEASE RESISTANT CROPS. In Proc. Soc. Prom. 

Agr. Sci., v. 28, p. 107-114. 



1908. THE CONSTANCY OF MUTANTS: THE ORIGIN OF DISEASE RESISTANCE 

IN plants. In Amer. Breeders' Assoc. Rpt., v. 4, [19071/08, p. 121- 
129. 



1908. [report of the] department of botany. In N. Dak. Agr. Exp. Sta., 
18th Ann. Rpt., [19073/08, pt. 1, p. 45-82. 



1909. SOME RESULTS AND OBSERVATIONS NOTED IN BREEDING CEREALS IN 

A specially prepared disease garden: In Araer. Breeders' Assoc. 
Rpt., v. 5, [i9o8]/o9, p. 177-182. 



191 1. [report of the] department of botany. In N. Dak. Agr. Exp. Sta., 
21st Ann. Rpt., [1910]/!!, pt, 1, p. 43-47. 



1912. [report of the] department of botany and plant pathology. In 
N. Dak. Agr. Exp. Sta., 22d Ann. Rpt., [i9ii]/i2, pt. 1, p. 23-60. 

(12) Brokema, L. 

1893. EENIGE WAARNEMINGEN EN DENKBEELDEN OVER DEN VLASBRAND. 

In Landbouwk. Tijdschr., 1893, p. 59-71, 105-128. 

(13) Eyre, J. V., and Smith, G. 

1916. SOME NOTES ON THE LINACEAE. THE CROSS POLLINATION OF FLAX. 

In Jour. Genetics, v. 5, no. 3, p. 189-197. 

(14) Gn.MAN, J. C. 

1916. CABBAGE YELLOWS AND THE RELATION OF TEMPERATURE TO ITS OCCUR- 
ENCE. In Ann. Mo. Bot. Gard., v. 2, no. 1, p. 25-84, 21 fig., 2 pi. 
Literature cited, p. 78-81. 

(15) Jones, L. R., and Gilman, J. C. 

1915. THE CONTROL OF CABBAGE YELLOWS THROUGH DISEASE RESISTANCE. 

Wis. Agr. Exp. Sta. Research Bui. 38, 70 p., 23 fig. Literature cited, 
p. 69-70. 

(16) Little, C. C. 

1914. A POSSIBLE MENDELIAN EXPLANATION FOR A TYPE OF INHERITANCE 

apparently non-mendelian in nature. In Science, n. s., v. 40, 
no. 1042, p. 904-906. 



Dec. 10, 191 7 Flaxwilt 605 

(17) Marryat, Dorothea C. E. 

1907. NOTES ON THE INFECTION AND HISTOLOGY OF TWO WHEATS IMMUNE TO 
THE ATTACKS OF PUCCINIA GLUMARUM, YELLOW RUST. In Jour. Agr 
Sci., V. 2, pt. 2, p. I29-I38, 2 pi. 

(18) Norton, J. B. 

I913. METHODS USED IN BREEDING ASPARAGUS FOR RUST RESISTANCE. U. S. 

Dept. Agr. Bur. Plant Indus. Bui. 263, 60 p., 4 fig., 18 pi. 

(19) Nypels, Paul. 

1897. NOTES pathologiques. In Bui. Soc. Roy. Bot. Belgique, t. 36, pt. 2, 
p. 183-276, 18 fig. 

(20) Orton, W. A. 

1909. THE DEVELOPMENT OF FARM CROPS RESISTANT TO DISEASE. In U. S. 

Dept. Agr. Yearbook, 1908, p. 453-464, pi. 39-40. 
(21) 

1913. THE DEVELOPMENT OF DISEASE RESISTANT VARITIES OF PLANTS. In 

Compt. Rend. 4th Internat. Conf. Genetique, 1911, p. 247-265, 9 fig. 

(22) Stuckey, H. P. 

1916. TRANSMISSION OF RESISTANCE AND SUSCEPTIBILITY TO BLOSSOM-END 

ROT in Tomatoes. Ga. Agr. Exp. Sta. Bui. 121, p. 83-91, 3 fig. 

(23) TlSDALE, W. H. 

1916. RELATION OF SOH, TEMPERATURE TO INFECTION OF FLAX BY FUSARIUM 

UNI. In Phytopathology, v. 6, no. 5, p. 412-413. 

(24) Vaughan, R. E. 

1914. A METHOD FOR THE DIFFERENTIAL STAINING OF FUNGUS AND HOST CELLS. 

In Ann. Mo. Bot. Gard., v. 1, no. 2, p. 241-242. 

(25) Ward, H. M. 

1902. ON THE RELATIONS BETWEEN HOST AND PARASITE IN THE BROMES AND 
THEIR BROWN RUST, PUCCINIA DISPERSA (ERIKSS.). In Ann. Bot., 
v. 16, no. 62, p. 233-315, 3 pi. 

(26) 

1905. RECENT RESEARCHES ON THE PARASITISM OF FUNGI. In Ann. Bot., V. 

19, no. 73, p. 1-54. Literature cited, p. 50-54. 



PLATE 44 

A. — Flax seedlings as grown in the tube culture for penetration studies: a, Seed- 
lings of susceptible flax in tubes inoculated with Fusarium lint, b, Susceptible flax 
seedlings growing in uninoculated tube, c, Resistant flax seedlings growing in un- 
inoculated tube, d, Resistant flax seedlings in tubes inoculated with F. lini. 
These seedlings were killed as readily as susceptible seedlings under these conditions. 

B. — Flax plants growing in North Dakota "flax-sick soil." a, Resistant flax 
No. 4. b, Susceptible No. 3. 

C. — Flax plants grown on soil from Madison, Wis. This soil was sterilized and 
inoculated with pure cultures of F. lint, a, Susceptible No. 3. b, Resistant 
No. 4. 

(606) 



Flaxwilt 



Plate 44 






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Journal of Agricultural Research 



Vol. XI, No. 11 



Flaxwilt 



Plate 45 




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Journal or Agricultural Research 



Vol. XI, No. 11 



PLATE 45 

A. — Flax plants growing on North Dakota "flax-sick soil." a, Resistant No. 4. 
b, c, Reciprocal crosses between resistant No. 4 and susceptible No. 6. All of 
these Fj plants died later of wilt, d, Susceptible No. 6. 

B. — Flax plants growing in North Dakota "flax-sick soil." a, Resistant No. 4. 
b, Cross between resistant No. 4 and susceptible No. 5. c, Susceptible No. 5. 

C. — Flax plants growing in North Dakota "flax-sick soil." a, Resistant No. 4. 
b, c, Crosses between resistant No. 4 and susceptible No. 5. d, Susceptible No. 5. 

D. — Flax plants growing in North Dakota "flax-sick soil." a, Resistant No. 4. 
b, Cross 4D20 between resistant No. 4 and susceptible No. 3. c, Cross 3E14 
between plants of the same two strains as b. d, Susceptible No. 3 (all dead). 



PLATE 46 

A. — Flax plants growing in soil inoculated artificially with Fusarium lini. a, 
Resistant No. 4. b, Susceptible No. 3. c-h, First-generation plants from crosses 
between these two strains. Notice that some of the crosses were almost entirely 
resistant to the disease while others wilted almost as badly as plants of the susceptible 
strain. 

B. — Second-generation flax plants growing on artificially infected soil, a, Sus- 
ceptible No. 3. b, Resistant No. 4. c-h, Second-generation plants from crosses 
between these two strains. These plants show the difference in individual plants of 
the first generation, c and d, e and /, g and h, respectively, came from individual 
plants of the first generation. 

C. — Second-generation flax plants growing in North Dakota "flax-sick soil." b, 
Susceptible No. 3. c, Resistant No. 4. a, d-h, Second-generation plants from 
the cross 4D20 in which the first generation were all resistant. Of the 530 plants of 
this F 2 generation 162 were resistant and 368 susceptible. 



Flaxwilt 



Plate 46 




Journal of Agricultural Research 



Vol. XI, No. 1 



